Occupant detection systems have the ability to determine the presence and/or characteristics of an occupant at a specific location. However, environmental conditions, such as humidity and moisture, may interfere with the occupant detection system's readings. Although useful in a wide variety of industries, occupant detection systems may be used in automotive applications to determine whether a person is present in a vehicle, and if so, the size of the person. This information may be used for various purposes, including enabling/disabling an airbag system. In many vehicles, each seat has at least one corresponding airbag.
Generally, during an accident, the airbags are deployed so long as a person is detected in the corresponding seat. For this reason, the occupant detection system further determines the person's size before enabling/disabling the airbag system. Although occupant detection systems are generally reliable, environmental conditions like humidity in the air or moisture on the seat may interfere with the occupant detection system's ability to determine the person's presence or size. For instance, the humidity or moisture may cause some children or car seats to be mischaracterized as adults.
Accordingly, an occupant detection system is needed that compensates for environmental conditions, such as humidity and moisture, which may interfere with the occupant detection system's readings.
Sensing devices are generally used to monitor existing conditions in order to control systems or to predict future conditions. A known problem, however, is that sensing devices are susceptible to environmental interference, and in some cases, are not able to tolerate environmental interference. One solution is to incorporate a compensation mechanism into the system to minimize or eliminate the interference. For electrically-interfaced sensing devices, the output of the sensing device may be an input into an electronic system that interprets and uses the sensor information. The electronic system may be affected by environmental interference such as electromagnetic interference (EMI) or electrostatic discharge (ESD). An electronic system often includes a filtering circuit to keep these and other environmental interferences from damaging the electronic system.
Although the filtering circuit may protect the electronic system from damage, the filtering circuit may still allow non-damaging electrical interference to pass into the system. The non-damaging electrical interference can affect the operation of the system by introducing error into the information obtained from the sensing device. In some cases, additional filtering circuitry may be used to reduce such errors, but this additional circuitry can introduce other errors which may be equally undesirable.
One option for overcoming this problem is to perform digital filtering on the electrical signal coming from the sensing device. Digital filtering on a signal is effective for reducing the impact of transient electrical interference. However, such filtering may not be able to detect long duration electrical interference. A method for being able to detect long duration electrical interference is to acquire a sequence of electrical signals from the sensing device, wherein the acquired signals represent the system response to operation at different frequencies and wherein the sequence of signals is acquired at substantially the same time. Because most electrical interference generally affects only bands of frequencies and not the whole frequency spectrum, signal changes at one frequency and not at other frequencies tend to indicate electrical interference within a certain frequency band. By reading a sequence of signals generated at different frequencies, long duration electrical interference can be detected and overcome.
Although the method of acquiring signals over multiple frequencies is effective to filter out long duration electrical interference, it cannot detect certain types of electrical interference that occur during the acquisition of the sequence of signals at different frequencies, or during certain other stages of the acquisition process.